When Richard Nixon won the presidency in 1968, in part on a promise to end the war in Vietnam, it was hard to find many on the American left who were surprised when the actual implementation of that plan contained a dramatic escalation—the aerial bombing of North Vietnam and U.S. military incursions into Cambodia—and four more years of carnage. Mr. Nixon, after all, was by no means the candidate of the political left, and the left held no illusions about his presidency.

Forty years later, the election of President Barack Obama was nothing if not an exercise in progressive illusion. Why else would one feel that sense of betrayal among progressives following Mr. Oba-ma’s Dec. 1 speech at West Point, accouncing his intention to engage 30,000 more American troops in the war in Afghanistan on top of the 32,000 already deployed?

This progressive disillusionment would not appear to have emanated from any special disingenuousness on the part of Mr. Obama during last year’s campaign. The president—as candidate—certainly presented as blank a slate as politically possible, allowing the electorate to fill it in with its own causes and colors. That’s simply smart politics. But anyone who paid any attention during the campaign would have known that while Mr. Obama was a legendarily early detractor to the war effort in Iraq, it was my impression that he said often enough that he did so to a certain degree because Iraq took America’s attention from where he felt the real terrorist threat emanated—Afghanistan.

Why the surprise, then, that Mr. Obama followed through on that premise to escalate the Afghani war?

That’s a question too difficult to answer in the context of a single column, and not necessary to resolve for the issue at hand. What is important for progressives to understand and acknowledge at this point is that Mr. Obama’s newly announced war measures, I believe, are not a betrayal of principles or a breaking of promises on the part of the president, but represent, rather, an inevitable divergence of the moderate-liberal-progressive national coalition that drove the Bush administration out of office.

The two overarching mandates of the American presidency—and of any American president who occupies the office—are to protect and preserve the U.S. Constitution, and to defend the national security.

Sometimes those two mandates come into direct and actual conflict, so that one could reasonably argue—President Abraham Lincoln certainly did—that Mr. Lincoln’s suspension of the right of habeus corpus—a fundamental Constitutional pillar—was necessary in the midst of Civil War to preserve the authority of any provision of the Constitution over the entire territory of the United States as that territory existed antebellum.

More often, though, the official definition of national security appears more nebulous, a moving target subject to the political inclinations of those doing the defining. Thus, the nation’s first president—George Washington—warned against the nation entangling itself in foreign wars, and for long periods during the next 150 years, it was more often than not conservative policy that the country was best protected by erecting a Fortress America that depended on two vast oceans to keep foreign invaders away.

The doctrine of American national safety through isolationism began to die in the 1940s with the Japanese attack on Pearl Harbor and the German U-boat scares up and down the East Coast, and ended completely with the invention of nuclear weapons deliverable by long-range missiles. But while World War II-era Japan and Nazi Germany actually attacked American home bases or states and Cold War-era Soviet Russia was certainly capable of it, the national security cover thereafter grew larger as the world grew smaller. Although not one of North Korea, North Vietnam, or Iraq possessed the capability of a mounting a military attack on the American mainland or even American-declared territories, American national security was invoked to justify American invasion of all three of those nations.

But while the North Korean, North Vietnamese, or Iraqi threats to America had to be explained on the basis of geopolitics or economic interests, the 2001 al Qaeda attacks on the World Trade Center and the Pentagon involved direct and immediate loss of human life and property. While al Qaeda posed and poses no threat of invasion along the lines of Axis Japan or Nazi Germany, or long-range nuclear annihilation as did Soviet Russia, there can be no doubt that the nation must protect itself and its citizens against such attacks.

Where progressives must draw the line with Mr. Obama is not over the issue of whether or not he should mount such a defense, but over the nature of what that defense ought to be.

It is tempting to argue that a considerable purpose of the Obama Afghani strategy is to help secure Mr. Obama’s re-election and the consolidation of the Democratic majority in the national legislature. That is strongly suggested in the core goals and timetables of the president’s strategy.

The core of that strategy was introduced in the middle of Mr. Obama’s West Point speech. “[A]s commander-in-chief, I have determined that it is in our vital national interest to send an additional 30,000 U.S. troops to Afghanistan,” the president said. “After 18 months, our troops will begin to come home. These are the resources that we need to seize the initiative, while building the Afghan capacity that can allow for a responsible transition of our forces out of Afghanistan.”

Mr. Obama went on to further define his strategic Afghani goals.

“Our overarching goal [is] to disrupt, dismantle and defeat al Qaeda in Af-ghanistan and Pakistan, and to prevent its capacity to threaten America and our allies in the future,” he said. “To meet that goal, we will pursue the following objectives within Afghanistan. We must deny al Qaeda a safe haven. We must reverse the Taliban’s momentum and deny it the ability to overthrow the government. And we must strengthen the capacity of Afghan-istan’s security forces and government so that they can take lead responsibility for Afghanistan’s future.”

That would be difficult enough, but thereafter, Mr. Obama introduced a bit of circular logic. Saying that American military escalation in Afghanistan should be matched in some manner by forces of members of the North Atlantic Treaty Organization, the president concluded that “taken together, these additional American and international troops will allow us to accelerate handing over responsibility to Afghan forces, and allow us to begin the transfer of our forces out of Afghanistan in July of 2011.”

In other words, the introduction of more troops into Afghanistan will allow the United States and its military allies to get its troops out of Afghanistan faster.

Certainly, an intelligent use of U.S.-NATO military forces could serve to disrupt al Qaeda bases and operation for some years to come, ensuring the nation’s security for a time. But that is not a solution that offers any semblance of permanence, only a sort of putting off of problems until a later date. And while saying that “the days of a blank check are over” and “we have no interest in occupying your country,” Mr. Obama gave no explanation as to why “July of 2011” should constitute some magic date upon which such a troop withdrawal ought to begin. It takes only a quick glance at the calendar to determine that the beginning of troop withdrawal in the summer of 2011 would mean the significant reduction of U.S. military operations in Afghanistan just in time for the spring, 2012 presidential primaries and fall national election.

While these little facts ought to be mentioned, they ought not to be the main thrust of the progressive criticism of Mr. Obama’s Afghanistan gambit.

Most progressives believe that America’s security can be established by non-military means, by using the nation’s wealth and technological capabilities to raise the economic standards of the rest of the world while allowing them to maintain their independence, culture and institutions. That’s a worthy goal. But it is difficult to win the trust and support of people in a nation we are in the process of invading. And it would be foolish to the point of insanity to believe that the operatives of organizations like al Qaeda would—under the present circumstances—allow a nonmilitary American presence in nations like Afghanistan and Pakistan without putting that presence under some form of violent attack.

Electoral considerations aside, Mr. Obama seeks to solve that dilemma—in the short run, at any rate—by the use of limited military force in Afghanistan. If progressives wish to provide a serious counter argument, they must develop a credible proposal as to what American security and national and world interests ought to be in countries like Afghanistan and Pakistan, and how those interests can be advanced in ways other than at the point of a gun.